There's still a decent amount of mature animation coming out for adults nowadays, it's just a bit harder to find, I think. Definitely not mainstream or screened in theaters like Isle of Dogs, which was fantastic, btw.
It's mostly stuff in the indie scene like A Cat in Paris, Mutafukaz, and Persepolis.
Actually, now that I think about it, all three of those are French films.
I think next to Japan, France is probably the second most interesting country when it comes to animation. Of course, they still produce kids' animation as well, such as Ladybug & Cat Noir, or other movies that seems to be directly inspired by Dreamworks output such as The Jungle Bunch or Zombillenium.
But then there's mature stuff like Crisis Jung and Mars Express which both heavily remind me of the golden age of OVAs.
Aside from that, there's plenty of arthouse-style works with their own unique style that have such a great amount of appeal to me.
Basically, what I'm saying is, don't sleep on French animation.